1. Field of Invention
The present invention relates to a semiconductor device and a method for forming the same. More particularly, the present invention relates to a high voltage device and a method for forming the same.
2. Description of Related Art
High voltage device, as implied by the name, is a device can be operated at relatively high voltage bias. That is, the breakdown voltage of the high voltage device is higher than that of the general device. Typically, in the high voltage device, the isolation layer is used to increase the distance between the source/drain region and the gate electrode or the lightly dopping ion dopping process is performed on drift region under the isolation layer and the grade region under the source/drain region so as to increase the junction breakdown voltage. Hence, under the high voltage, the high voltage device can be operated normally.
FIG. 1A is a cross-sectional view showing a conventional high voltage device. A high voltage device is located on the substrate 100. First, a polysilicon gate structure 103 is formed, and then a double diffused drain (DDD) structure 104 is formed in the substrate 100. The DDD structure 104 comprises a source/drain region 104a and a grade region 104b. The grade region 104b is used to alleviate the hot electron effect to enhance the breakdown voltage of the source/drain region 104a. 
FIG. 1B is a cross-sectional view showing another conventional high voltage device. In FIG. 1B, the elements are labeled with the reference number as same as that used to label the same elements shown in FIG. 1B. Among the high voltage devices shown in FIG. 1B, an isolation structure 106 is located between the gate structure 103 and the source/drain region 104a. Under the isolation structure 106, there is a drift region 108. The isolation structure 106 and the drift region 108 are used to alleviate the hot electron effect so as to increase the breakdown voltage of the source/drain region 104a. 